Diaryldiketopyrrolopyrroles of the formula ##STR2## wherein A.sub.1 and A.sub.2 are aryl radicals are well-known as important pigments.
U.S. Pat. No. 4,585,878 discloses N-substituted derivatives of the diaryldiketopyrrolopyrrole pigments wherein the N-substituent does not confer solubility in water. According to U.S. Pat. No. 4,585,878, the N-substituted derivatives of the diaryldiketopyrrolopyrrole pigments are prepared by reacting a diaryldiketopyrrolopyrrole compound in an organic solvent with a compound containing the ultimate N-substituents attached to a leaving group, or by reacting 2 moles of a compound of the formula R--N.dbd.CH--A, wherein R is the N-substituent and A is an aryl group, or one mole each of two different compounds of the formula R--N.dbd.CH--A, with a succinic acid diester in the presence of a base and an organic solvent and then dehydrogenating the product.
The present invention relates to a process for preparing a variety of N-substituted derivatives of diaryldiketopyrrolopyrrole pigments wherein the N-substituents are linked to the diarldiketopyrrolopyrrole by --CH.sub.2 -- or --O--CH.sub.2 -- linkages. The inventive process involves reacting a diaryldiketopyrrolopyrrole with formaldehyde to yield a 2,5-bis(hydroxymethyl)-3,6-diarylpyrrolopyrrole-1,4-dione intermediate and further reacting the intermediate, with or without isolation, with a second reactant which reacts with the hydroxymethyl groups. The products of the reaction are useful as colorants and as rheology improving agents for pigment dispersions. The 2,5-bis(hydroxymethyl)-3,6-diarylpyrrolopyrrole-1,4-dione intermediate is a useful as a synthon for the preparation of a variety of DPP derivatives, and as a stabilizer for polymers.